This invention relates to heat exchangers, particularly heat exchangers formed of reaction sintered silicon nitride or reaction sintered silicon carbide. Such heat exchangers would be useful in high temperature applications above 2000.degree. F. for conserving or exchanging heat energy. A particular application for my improved heat exchanger is a recuperator in gas turbine engines for aircraft or land vehicles, e.g. military tanks. The reaction sintered silicon nitride or carbide is desirable for such high temperature applications because of its small dimensional change on firing, and low creep or other physical changes at high operating temperatures. Unfortunately sintered silicon carbide or nitride articles, as conventionally formed, exhibit relatively high porosity. If used for heat exchangers such silicon carbide or silicon nitride materials would undesirably form leakage paths through the pores, thus rendering the exchangers useless for their intended purpose.
My invention relates to means and methods for sealing the pores in silicon carbide or silicon nitride wall structures, whereby said silicon structures can be formed into useful heat exchangers for high temperature applications. Prior to my invention others have attempted to seal off the undesired porosity in silicon carbide or silicon nitride wall structures. One prior art method that I am aware of involves the chemical vaporization and deposition of silicon carbide on a porous silicon carbide or silicon nitride substrate. However, if the deposited coating is thick enough to form a satisfactory seal for the porosity then it tends to wholly or partially clog the relatively small channels or passages in the heat exchanger, thus rendering the exchanger unsuitable as a fluid flow mechanism.
I contemplate a method of sealing off the undesired porosity which involves the application of two chemical reactants in gaseous form onto opposite surfaces of a heat exchanger constructed out of silicon carbide or silicon nitride. I believe that when the reactant gases penetrate into the pores of the heat exchangers they will contact each other within the individual pores; at that point in time they will chemically react to form a reaction product that will remain in the pores of the silicon carbide or silicon nitride substrate. The reactants are preselected so that the reaction product is chemically the same material as the base material, i.e. silicon nitride or silicon carbide. In the case of silicon nitride heat exchangers I propose to use as one reactant material gaseous ammonia, and as the other reactant material either silicon chloride or silicon hydride. The reaction product in that case is crystalline silicon nitride. In the case of silicon carbide heat exchangers I contemplate using as reactants methyltrichlorosilane and hydrogen.